This invention relates to machines for extruding thermoplastic material, and especially to vertical-axis extruding machines for forming an annular column of plastic material that progresses in a downward direction. The column of material is generally known as a parison. More particularly, the invention relates to an apparatus for changing the die sections that are used to form the parison.
In the science of blow-molding hollow plastic articles, a tubular parison is extruded from an extrusion head in a semisoft condition and at an elevated molding temperature. The parison is formed between a pair of closable blow-molding sections movable between open and closed positions and which define an inner surface that conforms to the desired outer surface configuration of the article to be molded.
The mold sections are closed on the parison to form an access opening around a blowing pin at one end of the mold and to pinch off and seal the parison at the other end of the mold. Air under pressure is admitted to the interior of the parison to expand it against the mold surface. When the molded article has cooled, the mold sections are separated and the article is removed.
The tubular parison is formed by extruding the thermoplastic material through an annular die passage defined by a pair of concentric die sections. These include an annular outer die section and a circular inner die section that defines with the outer section the annular extruding passage.
Because the same blow-molding apparatus may be used to form several different types of hollow plastic articles, it is frequently necessary to change the die sections. This is a delicate operation because the die sections weigh several hundred pounds, are usually quite hot, and often have residual plastic material within the extruding passage.
The outer die section is usually mounted by threaded fasteners to the housing of the extrusion apparatus, so it may be removed merely by removing the threaded fasteners while supporting it on a handling apparatus, such as a forklift truck. The inner die section, however, presents different problems, since it usually has a threaded rod on its inner end that is seated in internal threads within the housing. Accordingly, as the inner die section is unthreaded, a vertical clearance must be provided to permit it to move downward until its release, after which it may drop into a handling apparatus.
The inner die section is usually provided with two or more pins that extend downwardly from the lower end of the die section and which may be used to lock the die section to a changing fixture with holes that match the pins. Accordingly, the changing fixture must be engaged with the lower portion of the die section and then rotated to break the thread lock and unthread it from the housing. Because the die sections are extremely heavy, often not, and partly frozen to the housing by partially cured thermoplastic material, die changing has been a difficult and timeconsuming procedure.
The die changing apparatus of the present invention, however, reduces the difficulties indicated above, and affords other features and advantages heretofore not obtainable.